1. Field of the Invention
The present invention relates generally to a mobile communication system, and more particularly, to a method and apparatus for receiving system information from a Base Station (BS) in a Mobile Station (MS) in a mobile communication system.
2. Description of the Related Art
In a mobile communication system, a BS transmits to an MS system information, which refers to various pieces of information required for communications between the BS and the MS, including information for channel establishment and a list of neighbor cells. The system information is carried in System Information Blocks (SIBs), generally 18 SIBs.
The BS can use one of two methods to notify the MS of a change in the system information, i.e. a change in the SIBs. In one of the methods, the BS periodically transmits a Master Information Block (MIB) and the MS monitors the MIB. The MIB transmission period is equal to the monitoring period. The MIB, which provides information required for transmission of the SIBs, includes information bits called an MIB value tag and SIB value tags. The MIB value tag indicates whether the system information has been changed and the SIB value tags indicate an SIB that has been changed.
The MIB or the SIBs can be broadcast on a Broadcast Control CHannel (BCCH).
In this method, the MS monitors the system information in the same period as the transmission period of the system information of the BS. Therefore, when the system information is changed, modification time information is not needed. The modification time information indicates a time when the changed system information is applied.
The other method is that the BS notifies the MS of a change in the system information when the change occurs, rather than periodically. This method requires the modification time information.
A brief description will now be made of how it is determined whether the system information has been changed by use of the value tags. The following description applies to both the methods.
If a first MIB value tag is 0 and a second MIB value tag is 0, this indicates that no change has occurred to the system information. If an SIB is changed, the MIB value tag is set to 1, to thereby notify the MS of a change in the system information. Each time the system information changes, the MIB value tag sequentially increases. The MIB value tag just notifies the MS of whether the system information has been changed, and does not indicate the changed system information. Hence, when receiving a different MIB value tag compared to the previous one, the MS identifies the changed system information by checking SIB value tags.
If the MIB value tag is changed to 1 in the above example, the MS determines that the system information has been changed. To find out the changed system information, the MS then reads each of the 18 SIB value tags and acquires the changed system information from an SIB corresponding to a changed SIB value tag.
FIG. 1 illustrates a signal flow for a conventional operation for receiving system information from an Evolved Radio Access Network (E-RAN) in a User Equipment (UE) in a mobile communication system.
Referring to FIG. 1, in the mobile communication system, an E-RAN 100 transmits an MIB with an MIB value tag to a UE 110 in step 120 and an SIB to the UE 110 in step 130. It is assumed that the MIB value tag is error-free and has an increased value and thus the UE 110 has read SIB value tags. To determine a changed SIB using a changed SIB value tag, the UE 110 checks an error in the received SIB in step 140. If the SIB has an error, the UE 110 neglects the SIB in step 150 and receives a new SIB in step 160. For the sake of convenience, a description of MIB transmission is not provided herein.
In step 170, the UE 110 checks errors in the SIB. If no errors are detected, the UE 110 buffers the SIB in a buffer in step 180. If an error is detected in the received SIB, the UE 110 neglects the SIB and repeats SIB reception until no errors are detected in the procedure of FIG. 1. The repetition causes a long time delay for the UE to receive normal SIBs and determine the changed system information. As a consequence, communications are delayed between the E-RAN and the UE.